The present invention relates to a mold for injection molding a thermoplastic part with thick and thin wall sections. The mold has a void inducing member projected into a mold cavity, so as to inhibit the formation of surface defects, such as sink marks, which are typically present in such complex parts when manufactured by conventional injection molding method.
In the case of a large-sized part such as a housing for a TV set or a bumper for an automobile, the part must have structural strength as well as excellent surface appearance and dimensional accuracy. In order to obtain structural strength suitable for large parts, it is preferable that the part be designed to include thick wall sections which will reinforce the strength of the part. Hence, the structural integrity of the molded part will not be solely dependent upon the strength of the resin. These conflicting requirements often result in the sacrifice of appearance for strength or vice versa due to the dynamics of the conventional injection molding process.
When a part having both thick and thin wall sections is injection molded, the molten resin in the interior of the mold cavity is cooled and solidified at a slower rate than the exterior surface. Due to this delay of cooling and solidification, the volumetric shrinkage of the resin is likely to be accumulated in the more interior sections of-the part. The accumulation of the volumetric shrinkage often causes the formation of sink marks deformations, and other defects (hereinafter called sink marks) which typically occur for example, on the surface of the thick wall sections. Consequently, it is difficult to obtain a high quality surface in the molded part.
Due to this phenomenon, in a conventional injection molding process, parts are designed to avoid thick wall sections. When a part with thick wall sections must be manufactured by the conventional injection molding process, a dwelling step immediately after the injection of a molten resin into the cavity of a mold is performed. In the dwelling step, a dwelling pressure is applied to the molten resin existing in the cavity of the mold to maintain the packing density of the injected molten resin. However, when a part has a complicated configuration, a gate sealing precedes the completion of dwelling. As a result, sufficient dwelling pressure cannot be applied to the molten resin at position which are disposed far from a gate. The resulting part will not have a satisfactory surface appearance.
Japanese Patent Application Publication 61-53208, Japanese Patent Application Laid Open 63-268611 and Japanese Patent Application Laid Open 64-63122 disclose methods using a high-pressure gas as an auxiliary means of aiding the conventional injection molding process to inhibit the formation of sink marks. These methods comprise a two step process to inhibit surface defects in a molded part. In the first step, the molten resin, in an amount insufficient to fill the cavity of the mold, is injected into the cavity. At the same time or after, high-pressure gas is injected into the molten resin to form a hollow portion through a resin flow passage. In the second step, the application of dwelling pressure derived from the high-pressure gas through the hollowed resin flow passage to the interior of the molten resin is maintained so that the molten resin in the cavity is cooled and solidified against the inner wall of the mold to inhibit the formation of surface defects.
However, when a part having a complicated configuration is to be manufactured, the thick wall portion(s) connected with a gate are likely to act as flow leaders for the molten resin, so that the supply of the molten resin to each portion of the mold cavity becomes unbalanced. The result of this unbalance is air traps and flow marks which cause surface defects on the part. In extreme cases, the high-pressure gas may break the surface layer of the solidifying resin part so that the injection molding cycle itself is interrupted and a defective part is manufactured. Moreover, there is a restriction on the design configurations of parts to which these methods are applicable. If a hollow portion sufficient to inhibit the formation of sink marks cannot be formed in the mold cavity, the formation of sink marks on the surface of the part will result. Thus, these methods are not satisfactory for insuring parts with superior surface appearance.
Japanese Patent Publication 48-41264 discloses another injection molding method using high-pressure gas as auxiliary means, when an article having a thick wall throughout the part is to be manufactured. In this method, after molten resin is injected into the cavity of a mold, a gas nozzle is directly projected into the molten resin in the cavity. High-pressure gas is supplied through the gas nozzle into the molten resin to perform the aforementioned first step while forming a hollow portion in the molten resin. This avoids the first step problems enumerated above. However, the configuration of a part to which this method is applicable is limited to uniformly thick wall sections. Moreover, this process requires a reciprocal carrying mechanism for inserting the gas nozzle into the molten resin during injection molding and withdrawing the gas nozzle from the solidified part. This method lacks in practicality, since it requires a driving mechanism with a withdrawing power larger than a constraining force derived from the shrinkage of the resin during solidification to remove the gas nozzle from the part.
Besides, these methods have significant problems such as the danger which accompanies the use and handling of high-pressure gas.
Japanese Patent Publication 61-9126 discloses the method wherein the cavity of a mold is filled with a molten resin, and the thick wall portion of the part is then pressed by a gas pressure from a position corresponding to the back side of the part. However, the direct application of the gas pressure causes the formation of irregular sink marks on the back surface of the part resulting in an unsatisfactory surface appearance.
When a resin such as polycarbonate or polymethyl-methacrylate, which set up quickly, is injection molded under conditions that permit the surface layer to rapidly cool and solidify, the formed surface layer of the part exhibits higher strength than the force derived from the volumetric shrinkage of the resin. Consequently, voids are formed in the thick wall portion of the part without deformation of the surface layer. In this case, sink marks are not formed on the surface of the thick wall portion.
The formation of these internal voids is regarded as a defect which causes a reduction of the strength in the part. However, this phenomenon of forming voids can be effectively controlled to inhibit the formation of sink marks, as disclosed in Japanese Patent Application Publication 2-13886. In this method, a void control member is projected through the inner surface of a mold into the cavity where sink marks are apt to form on the surface of the part. The void control member promotes the formation of a void in the resin body near the top end of the void control member, thereby inhibiting the formation of sink marks. This method has the advantage that the formation of sink marks can be inhibited by a simple, economical means.
However, the void control means must be made of a material having large heat capacity, so as to be held at a higher temperature in order to spontaneously form voids. Consequently, it is difficult to make the void control members smaller in size. The position where the void control members may be located in the mold are restricted, and the void control members cannot be held at a high temperature under a stable condition. As a result, it is difficult to ensure the thick wall sections of the part will be free of sink marks.
In addition, the positions where sink marks will be likely to be formed are irregularly affected by the molding conditions. For instance, the sink marks are formed at different positions during every molding cycle, even when the same part is manufactured. In this regard, it is difficult to predetermine the position where sink marks will be formed. The size and the configuration of the thick wall portion has an influence on the positions where sink marks are to be formed as well. Furthermore, in some resins, such as polycetal, the voids do not expand to the extent sufficient to compensate for the volumetric shrinkage of the resin, so that the formation of sink marks on the surface of a part cannot be completely eliminated.